This invention relates to the art of reproducing phonograph records, and more particularly, to an automatic two-speed changing device for playing records designed to be reproduced at different speeds of rotation.
Coin-operated phonographs, commonly referred to as "juke boxes", for some years have been designed to reproduce or play records designed for rotation at a single speed such as 45 rpm. These records are 7 inches in diameter and they have a relatively large center hole of approximately 11/2 inches. However, recently records of similar size have been produced to be played at 331/3 rpm, these records commonly being referred to in the art as "little LPs". These little LPs have small center holes and are thereby readily distinguished by their appearance from the 45 rpm records. It is often desired to place both 331/3 and 45 rpm records in a juke box for reproduction alternately. It is known that the different size center holes can be used to control speed devices to change the speed of the records. Such devices are disclosed in the patents to Osborne U.S. Pat. Nos. 2,673,089 and 2,851,273.
In general, the prior art approach of changing the speed of rotation of intermingled 45 and 331/3 rpm records is rather complex and expensive. Many prior art devices incorporate lock, trip and release mechanisms to change the mechanical coupling ratio between the drive motor and the turntable. This is done either by changing the ratio of pulleys or by changing drive wheel diameters as is wellknown in the art.
Two basic problems are inherent with prior art devices. First, the 45 rpm pilot assembly must be reset between record plays. Presently, a trip mechanism removes a support member from beneath the 45 rpm pilot and the pilot falls out of the way by gravity. The 45 rpm pilot then must be reset before it can accept a 45 rpm record. Secondly, the pilots must be designed to accept records which have diameters that can vary as much as 1/16 inch undersize from the normal record hole dimension and further enable the record to be placed in a playing position without the possibility of bending or cracking the record. Furthermore, records which have holes larger than the desired dimension must be accurately centered to eliminate or minimize the "wow" which may develop during playing.
Accordingly, it is an object of this invention to provide a new and improved two-speed automatic phonograph which overcomes the problems of the prior art by providing means for receiving records having different hole dimensions and centering the record properly.
Still another object of this invention is to provide a two-speed automatic phonograph which eliminates the need for mechanical speed-changing devices.
Another object of this invention is to provide a two-speed phonograph which includes means for sensing the characteristic of the record being played, i.e. either 331/3 rpm or 45 rpm, and automatically resetting itself after being set by one of the records.
Still another object of this invention is to provide a new and improved two-speed phonograph which has improved starting torque characteristics and reduced "hum" and "wow" during playing of records.
Briefly, the present invention provides a new centering structure which has substantially improved operating characteristics and which is relatively inexpensive to manufacture. The record-centering structure utilizes the ability of a pivoted arm to resist extremely high forces applied radially toward its pivot point. yet will yield relatively easily to extremely low forces applied tangentially to its pivot point. The centering mechanism incorporates three record guides which are positioned in the center portion of a turntable and arcuately spaced apart from one another 120.degree.. Each of the guides includes an arm portion extending therefrom and pivot means formed on the arm portion to be pivotally secured beneath the turntable. In the illustrated embodiment, the pivot is formed by two 0.015 inch diameter spherical portions formed on conical protuberances which fit into correspondingly shaped sockets. This enables the guide to rotate about its pivot point. Spring pressure is exerted to hold the guide in a position to receive a 45 rpm record. However, when a record of the 331/3 rpm type is applied to the turntable, the weight of the record will push down on the guide causing it to pivot about its pivot point and allow the record to rest upon a smaller diameter spindle for centering.
The automatic phonograph of this invention also includes means for providing a high starting voltage to a drive motor to generate starting torque and thereafter reduces the voltage to the drive motor during normal running speed. This reduced voltage also reduces the "hum" and "wow" characteristics often encountered in phonograph apparatus of the type disclosed herein. Preferably, a synchronous motor is utilized as a drive motor and is operated to provide the appropriate playing speed of the record by controlling the synchronous motor with an AC signal of predetermined frequency. To change the speed of the motor, the frequency of the AC signal is changed. This is accomplished, in the illustrated embodiment, automatically by providing a light responsive sensing circuit which controls the operation of an oscillator which, in turn, controls the speed of rotation of the synchronous motor.
Many other objects, features and advantages of this invention will be more fully realized and understood from the following detailed description when taken in conjunction with the accompanying drawings wherein like reference numerals throughout the various views of the drawings are intended to designate similar elements or components.